1. Field of the Invention
The present invention relates to a valve train of an internal combustion engine that includes a valve operating cam for opening and closing engine valves and a drive cam for driving the valve operating cam for controlling the timing of the opening and the closing of the engine valves.
2. Description of Background Art
A valve train for an internal combustion engine is known as described below. The valve train includes a drive cam rotated integrally with a camshaft supported by a cylinder head and a valve operating cam swingably supported by the camshaft to open and close the engine valves. In addition, the valve train includes a link mechanism supported swingably around the camshaft to transmit a valve driving force of the drive cam to the valve operating cam for swing and a drive mechanism for swinging the link mechanism around the camshaft. The valve train configured as above can vary the operative characteristics of the engine valves depending on the swinging position of the swung link mechanism. See, for example, Japanese Patent Laid-open No. 2005-233180.
In the traditional valve train described above, a sub-rocker arm constituting the link mechanism is lifted and swung by the drive cam. The drive cam is provided with a base circular portion adapted not to swing the sub-rocker arm and a cam lobe portion adapted to swing the sub-rocker arm as well as with a buffer portion at a section transiting from a base circle to a cam lobe. FIG. 12 is a graph showing the operating characteristics of the traditional drive cam. The traditional buffer portion S is described with reference to FIG. 12.
In general, the base circular portion and the cam lobe portion are not smoothly joined to each other. In the drive cam of FIG. 12, the buffer portion S includes a buffer transition section Sa where lift speed is increased along with the rotation of a camshaft and a buffer constant-speed section Sb being continuous with the buffer transition section Sa and providing constant lift speed. An acceleration section Sc adapted to increase the lift speed at high acceleration is provided continuously with the buffer constant-speed section Sb.
Since the buffer transition section Sa provides relatively high lift acceleration, it is probable that the sub-rocker arm is elastically deformed and allowed by the buffer transition section Sa to jump slightly. Also if the sufficient length of the buffer constant-speed portion Sb cannot be ensured, the behavior of the sub-rocker arm probably may become unstable.
Further, if the buffer transition section Sa and the buffer constant-speed section Sb are provided, the section of the acceleration section Sc adapted to increase the swing speed of the sub-rocker arm cannot be lengthened according to such a provision. Therefore, the acceleration of the sub-rocker arm is increased at the acceleration section Sc. More particularly during the high-rotation of the internal combustion engine, the elastic deformation of the sub-rocker arm may be increased and the jumping of the sub-rocker arm may occur. Thus, the swing amount of the sub-rocker arm varies, which probably influences the operating characteristics of the engine valve.